That oil tank via the furnace must have been drawing a fair amount of current, couple of amps maybe. An underground oil tank has a large surface area touching the dirt, more so than a grounding rod. They should see a noticeable drop in their electric bills after you found it.
How do you correct the overloaded circuit? Run dedicated lines to the air conditioners? And leave the old circuit to just feed table lamps and such small loads? Or find a place to split the old circuit, and run a new line to feed the far end half of the old circuit (after cutting the old circuit in half)?
